1. Field of the Invention
The present invention concerns a tuning system for a television receiver with automatic fine tuning (AFT) provisions.
2. DESCRIPTION OF THE RELATED ART
Most television receivers include circuitry for generating an analog AFT signal the polarity and the magnitude of which represent the polarity and magnitude of deviation of the frequency of the amplitude molulated picture carrier of the IF signal from its nominal frequency value. The AFT signal is applied to the local oscillator of the tuner of the television receiver to reduce the frequency deviation of the IF picture carrier.
Originally, the AFT signal was used to correct for the drift of the local oscillator signal due to temperature and aging. Closed loop phase and frequency locked tuning systems (often known as frequency synthesis tuning systems), which "lock" the frequency of the local oscillator signal to a reference frequency, obviate the need for an AFT signal for the above-stated purpose. However, an AFT signal is typically employed in frequency synthesis tuning systems to correct for frequency deviations of the IF picture carrier due to corresponding frequency offsets between the picture carriers of the RF signals provided by some television signal sources such as cable systems, video tape and disk players, home computers and cameras, and nominal frequency values as specified by broadcast standards.
In one type of frequency synthesis tuning system with AFT provisions, such as is disclosed in U.S. Pat. No. 4,405,947 issued to J. Tults and M. P. French on Sept. 30, 1983, after the frequency of the local oscillator signal is locked to the nominal local oscillator frequency for a selected channel by a phase locked loop, the analog AFT signal is coupled to the local oscillator through the lowpass filter of the phase locked loop to control the local oscillator directly to correct for RF signal frequency offsets. Because the lowpass filter has to filter the AFT signal as well as the output signal of the phase comparator of the phase locked loop, the filter may be complex in order to ensure proper AFT response time and sensitivity throughout the relatively wide television tuning range.
In another type of frequency synthesis tuning system with AFT provisions, such as described in U.S. Pat. No. 4,302,778 issued to A. Tanaka on Nov. 24, 1981, the AFT signal is not coupled directly to the local oscillator but rather is used to change the frequency of the local oscillator in small steps by controlling the division factor of a programmable division factor of the phase locked loop comprising the frequency synthesizer. In such a system, since the AFT signal is not filtered by the lowpass filter of the phase locked loop, the design constraints of the filter can be relaxed.